This renewal grant request is to provide support for research designed to elucidate the energy sources supporting sodium reabsorption in the mammalian renal nephron. The protocols outlined in this grant proposal are designed particularly to study the role of transepithelial solute gradients, and of intraluminal and peritubular composition on proximal tubular filtrate reabsoption. The techniques to be used are micropuncture collection and analysis of tubular fluid and peritubular capillary blood, in vivo microperfusion of the proximal tubule and the peritubular capillary blood supply, microanatomical analysis of tubular and vascular patterns in the methyl salicylate cleared, silicone rubber injected rat kidney, and in vitro microperfusion of the proximal convoluted tubule of the rabbit.